POS0418 SREBP2 REGULATES OSTEOCLAST DIFFERENTIATION AND PROTECTS MICE FROM INFLAMMATORY BONE LOSS
Authors: H. Kim, I. A. Choi, S. Bae, K. Kaneko, E. Giannopoulou, L. Deng, K. H. Park-Min
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> Bone erosion is a key clinical feature of inflammatory osteolysis. Emerging evidence shows a link between biological lipids and the pathogenesis of bone loss in inflammatory conditions. Osteoclasts are the sole bone resorbing cells and are responsible for bone erosion. Sterol regulatory element binding protein 2 (SREBP2) is a key transcription factor for cholesterol biosynthesis and a sensor for intracellular lipids. <h3>Objectives</h3> We aim to test the role of SREBP2 in physiological and pathological bone resorption. <h3>Methods</h3> Lysozyme M promoter-driven Cre transgene on the C57/BL6 background mice and Srebp<sup>flox/flox</sup> mice were crossed to generate Srebp2<sup>fl/fl</sup>LysM-Cre (referred to as SREBP2<sup>ΔM</sup> mice). Micro-computed tomography (µCT) analysis was performed to analyze bone volume and architecture. Mouse bone marrow-derived macrophages were cultured with receptor activator of nuclear factor-kappa beta ligand (RANKL) and macrophage colony stimulating factor (M-CSF)-containing conditioned media in α-minimum essential medium supplemented with 10% fetal bovine serum for three days to generate osteoclasts. For the supracalvarial osteolysis model, recombinant human tumor necrosis factor-α (TNFα, 40µg/kg) was injected daily into calvarial periosteum in 12-week-old female control and SREBP2<sup>ΔM</sup> mice for five consecutive days. <h3>Results</h3> µCT analysis of the femurs of 12-week-old female mice revealed a significant decrease in trabecular bone mass of SREBP2<sup>ΔM</sup> mice relative to control mice. In contrast, cortical parameters, including cortical bone volume/tissue volume (BV/TV) and porosity, were comparable between the two groups. Bone histomorphometric analysis of the femur indicated that the trabecular osteoclast numbers, surface area, and bone resorption activity were significantly higher in SREBP2<sup>ΔM</sup> mice than in control mice. However, there was no difference in mineral apposition rate and bone formation rate between SREBP2<sup>ΔM</sup> mice and control mice. Consistent with in vivo findings, RANKL-induced in vitro osteoclast differentiation was accelerated in SREBP2-deficient cells compared to control cells, as measured by the formation of TRAP+ multinucleated cells. Moreover, in TNF⍺ and RANKL co-treatment settings, SREBP2 deficiency enhanced TNF⍺-mediated enhancement of osteoclastogenesis compared to control cells. In the supracalvarial osteolysis model, TNF⍺ administration over calvaria increased bone resorption pit area in SREBP2<sup>ΔM</sup> mice compared to littermate control mice. µCT analysis revealed a significant decrease in calvaria bone volume/tissue volume and apparent density of SREBP2<sup>ΔM</sup> mice compared to control mice. The histomorphometric analysis demonstrated significant increases in trabecular osteoclast numbers and surface area in SREBP2<sup>ΔM</sup> mice relative to littermate control mice, suggesting that inflammatory osteolysis is enhanced under SREBP2 deficiency. <h3>Conclusion</h3> Using the myeloid-specific SREBP2-deficient model, we demonstrated that SREBP2 negatively regulates in vitro and in vivo osteoclastogenesis. SREBP2-deficient mice exhibit an osteoporotic phenotype. SREBP2 deficiency exacerbates TNF⍺-induced osteolysis in mice, supporting a protective role of SREBP2 in inflammation-associated pathological bone loss. <h3>REFERENCES:</h3> NIL. <h3>Acknowledgements:</h3> NIL. <h3>Disclosure of Interests</h3> None Declared.